Jam

Unopened jam keeps 1–2 years in the pantry. Once opened, refrigerate it — it keeps about 6–12 months.

Always use a clean, dry spoon. This is the whole discipline with jam. The sugar concentration protects the jam, but crumbs, butter, and peanut butter introduced on a knife create local pockets of diluted sugar where mold grows readily. A jar of jam almost always fails from what was put into it.

Wipe the rim before closing. Sticky residue on the thread is the second most common mold site.

Reduced-sugar and no-sugar-added jams keep a much shorter time — often just 3–4 weeks opened — because the preservative has been removed.

Jam freezes fine, though there is rarely reason to.

Jam is preserved by sugar, and the mechanism is worth understanding because it explains everything else about how jam behaves.

Sugar at high concentration binds water molecules, lowering the water activity of the jam to roughly 0.75–0.80. Bacteria need about 0.85 to grow and cannot survive there, which is why jam has never been a vehicle for bacterial foodborne illness. Fruit acidity, usually pH 3.0–3.5, adds a second barrier.

Molds and osmophilic yeasts, however, tolerate low water activity far better than bacteria do, which is why mold on the surface is the characteristic failure of an old jar.

The USDA's advice here is stricter than folk practice: discard the whole jar rather than scooping off the moldy layer. Jam is high in moisture and soft, so filaments extend invisibly beneath the surface, and some molds produce mycotoxins that diffuse into the surrounding food. The old habit of scraping the top off is not supported by current guidance.

Reduced-sugar jams have less protection by design and rely on refrigeration and preservatives, so their shorter window is real rather than cautious labeling.

Other signals: fermentation producing bubbles or an alcoholic smell, a domed lid, and liquid weeping out with an off odor. Sugar crystallization and slight darkening are normal.